1. Field of the Invention
This invention relates to tissue ablation systems. More particularly, this invention relates to monitoring of contact between an invasive probe and tissue within the body.
2. Description of the Related Art
Cardiac arrhythmias, such as atrial fibrillation, occur when regions of cardiac tissue abnormally conduct electric signals to adjacent tissue, thereby disrupting the normal cardiac cycle and causing asynchronous rhythm.
Procedures for treating arrhythmia include surgically disrupting the origin of the signals causing the arrhythmia, as well as disrupting the conducting pathway for such signals. By selectively ablating cardiac tissue by application of energy via a catheter, it is sometimes possible to cease or modify the propagation of unwanted electrical signals from one portion of the heart to another. The ablation process destroys the unwanted electrical pathways by formation of non-conducting lesions.
Verification of physical electrode contact with the target tissue is important for controlling the delivery of ablation energy. Attempts in the art to verify electrode contact with the tissue have been extensive, and various techniques have been suggested. For example, U.S. Pat. No. 6,695,808 describes apparatus for treating a selected patient tissue or organ region. A probe has a contact surface that may be urged against the region, thereby creating contact pressure. A pressure transducer measures the contact pressure. This arrangement is said to meet the needs of procedures in which a medical instrument must be placed in firm but not excessive contact with an anatomical surface, by providing information to the user of the instrument that is indicative of the existence and magnitude of the contact force.
As another example, U.S. Pat. No. 6,241,724 describes methods for creating lesions in body tissue using segmented electrode assemblies. In one embodiment, an electrode assembly on a catheter carries pressure transducers, which sense contact with tissue and convey signals to a pressure contact module. The module identifies the electrode elements that are associated with the pressure transducer signals and directs an energy generator to convey RF energy to these elements, and not to other elements that are in contact only with blood.
A further example is presented in U.S. Pat. No. 6,915,149. This patent describes a method for mapping a heart using a catheter having a tip electrode for measuring local electrical activity. In order to avoid artifacts that may arise from poor tip contact with the tissue, the contact pressure between the tip and the tissue is measured using a pressure sensor to ensure stable contact.
U.S. Patent Application Publication 2007/0100332 describes systems and methods for assessing electrode-tissue contact for tissue ablation. An electromechanical sensor within the catheter shaft generates electrical signals corresponding to the amount of movement of the electrode within a distal portion of the catheter shaft. An output device receives the electrical signals for assessing a level of contact between the electrode and a tissue.
U.S. Pat. No. 7,306,593, issued to Keidar et al., describes a method for ablating tissue in an organ by contacting a probe inside the body with the tissue to be ablated, and measuring one or more local parameters at the position using the probe prior to ablating the tissue. A map of the organ is displayed, showing, based on the one or more local parameters, a predicted extent of ablation of the tissue to be achieved for a given dosage of energy applied at the position using the probe. The given dosage of energy is applied to ablate the tissue using the probe, and an actual extent of the ablation at the position is measured using the probe subsequent to ablating the tissue. The measured actual extent of the ablation is displayed on the map for comparison with the predicted extent.
U.S. Patent Application Publication Nos. 2008/0288038 and 2008/0275465, both by Sauarav et al., which are herein incorporated by reference, describe an electrode catheter system, which may comprise an electrode adapted to apply electric energy. A measurement circuit adapted to measure impedance may be implemented between the electrode and ground as the electrode approaches a target tissue. A processor or processing units may be implemented to determine a contact condition for the target tissue based at least in part on reactance of the impedance measured by the measurement circuit. In another embodiment, the contact condition may be based on the phase angle of the impedance.
The document Determinants of Lesion Size During Radiofrequency Catheter Ablation: The Role of Electrode-Tissue Contact Pressure and Duration of Energy Delivery, David E. Haines, J. Cardiovasc Electrophysiol, Vol. 2, pp. 509-515, December 1991 described the effects of varying electrode tissue contact pressure and duration of RF energy delivery on the size of the resultant lesion in an in vitro model of canine myocardium. A greater contact force significantly decreased the power required to maintain a constant electrode-tissue interface temperature, and the lesion size grew monoexponentially with time.
U.S. Patent Application Publication No. 2010/0298826 describes a force-time integral for real time estimation of lesion size in catheter-based ablation systems. The apparatus measures the force exerted by a contact ablation probe on a target tissue and integrates the force over an energization time of the ablation probe. The force-time integral can be calculated and utilized to provide an estimated lesion size (depth, volume and/or area) in real time. The force-time integral is said to possibly account for variations in the power delivered to the target tissue in real time to provide an improved estimation of the lesion size.